1. Field of the Invention
The present invention relates to examination probes and, more particularly, to ultrasonic probes.
2. Related Art
Non-destructive inspection is commonly used in the aircraft industry to inspect aircraft structures for any type of internal or external damage to the structure. Among the structures that are routinely non-destructively tested are composite structures.
An ultrasonic probe may be used to examine structures by transmitting ultrasonic signals to the structure and measuring a reflected signal returned by the structure to the probe. To facilitate the transmission of signals between conventional probes and the structure being examined, the structure is often wetted where the probe is to be applied with a coupling fluid (a “couplant”), such as water. The ultrasonic signals more easily transfer from the transducer to the structure by way of the couplant disposed between the probe and the structure.
When examining structures, it is often desired to examine edges of the structure. However, when a portion of conventional probes is moved over an edge, any coupling fluid that was disposed beneath the portion of the probe moved beyond the edge falls over the edge leaving no coupling fluid adjacent that portion. When coupling fluid is absent from the over-the-edge portion of the probe, the fluid between the probe and the item adjacent the edge will tend to also flow over the edge. When this happens, the probe is typically unable to accurately determine characteristics of the item adjacent the edge.
In one particular example, a delay line may be used in order to provide an edge inspecting capability. The delay line is typically a material, such as Rexolite or Plexiglass, which has acoustic impedance that is not the same as water. In operation, the probe is in contact with the delay line and the coupling fluid is applied between the probe and the delay line. The distance between the delay line and the surface of the part being inspected is normally between about 0.020″ to 0.030″. Water passageways are created, such as a series of small holes formed along the length of the delay line, to allow the coupling fluid to enter between the delay line and part surface. The surface tension of the coupling fluid is used to keep coupling fluid under the delay line as the probe traverses over the edge of the part surface.
This operation has many disadvantages. For example, the inspection speeds are slow, on the order of about 10 inches per second. Moreover, only “smooth” surfaces may be properly inspected, since it is difficult to maintain the coupling fluid over a rough surface. Finally, the acoustic mismatch between the delay line and the coupling fluid may alter the inspection frequency of the probe.
What is needed is an inspection apparatus and associated inspection method that improves the coupling of probes to items being examined allowing reliable, expedient, and accurate evaluation of item characteristics including at their edges.